1. Field of the Invention
This invention relates to an image signal recording and reproducing system for recording an image signal on a recording medium and for reproducing the image signal recorded on the recording medium.
2. Description of the Related Art
A still video system (hereinafter referred to as the SV system) has been known as an apparatus for recording and reproducing a still image signal.
The SV system is arranged to frequency-modulate the current TV signal of, for example, the NTSC system and to record the frequency-modulated TV signal on a magnetic disc called a video floppy disc. Therefore, the image signal recorded and reproduced by the SV system has been arranged to have a degree of resolution which approximately conforms to the current TV system.
On some occasions, a picture reproduced by a still-image signal handling system like the SV system is printed out. However, the quality of the printed picture (particularly, the resolution of it) is inferior to a sliver-halide type photograph.
Meanwhile, new TV systems such as a high-definition TV (hereinafter referred to as HDTV) system have recently come to be studied. The HDTV system has about 1000 scanning lines which are about twice as many as those of the current NTSC system and also has a horizontal signal band matching with those many scanning lines.
Therefore, it has become necessary to improve the SV system into a still image recording/reproduction system capable of giving about the same resolution as that of the HDTV system or the like which has 1000.times.1000 picture elements (for a square picture plane).
However, use of a new recording format for the purpose of recording a still image of such a high picture quality that has 1000.times.1000 picture elements (for a square picture) would make the still image record thus obtained unreproducible by the reproducing device of the conventional SV system. To solve this interchangeability problem, the following method has evolved.
This method is called a CHSV (compatible high-definition still video) system. The CHSV system not only permits still image signals to be recorded and reproduced with the same degree of resolution as the HDTV system but also gives the interchangeability with the conventional SV system.
According to the CHSV system, for example, a luminance signal which has been offset-sampled at intervals of time Ts as shown in FIG. 1 of the accompanying drawings is first divided into four field image planes Ai, Bi, Ci and Di (i: 1, 2, . . . ) and recorded in four tracks on a video floppy disc in accordance with the current SV format. In reproducing, the reproduced signal is re-sampled to restore the offset-sampled luminance signal. After that, the signal thus restored is stored in a frame memory. Then, a picture element interpolation process is carried out with picture elements stored in the frame memory. By this, an image signal having a degree of resolution of about 1300.times.1000 picture elements can be reproduced and output by the CHSV system.
The above-stated CHSV system performs "analog transmission of sampled values" in a manner as shown in FIG. 2 through a transmission path consisting of frequency-modulating, frequency-demodulating and electro-magnetic converting elements and by performing re-sampling at the time of reproduction. In actuality, however, a series of sample values to be supplied to the transmission path is not in an impulse state as shown in FIG. 2 but has a finite width as shown in FIG. 3. Since the transmission path has a low-pass filter characteristic (Nyquist characteristics), it is necessary, for accurate restoration of the signal, to make aperture correction for the signal after it has passed through the transmission. path as shown in FIG. 3.
Further, in order to correctly carry out the analog transmission of sample values, it is necessary (i) that the transmission path has a low-pass filter characteristic and (ii) that the re-sampling process is correctly performed on the signal receiving side.
If the above-stated condition (ii) is not satisfied, in particular, ringing would take place near the edge part of the image to deteriorate the quality of the image reproduced. In view of this, for correct re-sampling on the signal receiving side, a phase reference signal which serves as a reference to the phase of re-sampling is added to the image signal to be transmitted. Then, on the signal receiving side, the phase of re-sampling can be controlled in accordance with the phase reference signal included in the image signal transmitted.
FIG. 4 is a block diagram showing in outline the arrangement of an image signal reproducing apparatus conforming to the CHSV system. To simplify the description, FIG. 4 includes only luminance signal reproducing blocks. Referring to FIG. 4, a signal recorded on a video floppy disc 501 is reproduced by a reproducing head 502. The reproduced signal is supplied to a reproduction processing circuit 503. The reproduction processing circuit 503 then performs frequency demodulation and deemphasis processes, etc., on the signal and outputs a reproduced image signal. After the circuit 503, the reproduced image signal is subjected to an aperture correction process which is performed by an aperture correction filter 505, to a clamping process which is performed by a clamp circuit 506 and is then supplied to an analog-to-digital (A/D) converter 507.
Meanwhile, a re-sampling clock signal generating circuit 504 separates, from the signal reproduced by the reproducing head 502, only a time base correcting pilot signal component which has been frequency-multiplexed with the image signal at the time of recording. A PLL (phase-locked loop) circuit disposed within the re-sampling clock signal generating circuit 504 then forms, on the basis of the pilot signal, a re-sampling clock signal fs which follows time base variations taking place in the reproduced image signal.
The re-sampling clock signal fs which is thus output from the re-sampling clock signal generating circuit 504 is supplied to a variable delay circuit 508. The variable delay circuit 508 controls the phase of the re-sampling clock signal and supplies it to the A/D converter 507. The phase controlling action of the variable delay circuit 508 on the re-sampling clock signal fs is performed under the control of a phase control signal generating circuit 509. The phase control signal generating circuit 509 is arranged to receive a re-sampled image signal from the A/D converter 507 and to form a phase control signal PC from the re-sampled image signal according to a sample value corresponding to the phase reference signal part thereof. The phase control signal PC thus formed is supplied to the variable delay circuit 508 to adjust the phase of the re-sampling clock signal to a correct phase.
The re-sampling clock signal which is thus adjusted to have a correct phase is supplied to the A/D converter 507 to be used for digitizing the reproduced image signal. The digital reproduced image signal thus obtained is Nyquist-equalized by an automatic equalizer 510.
The automatic equalizer 510 is formed by a digital filter. The equalizing characteristic of the automatic equalizer 510 is arranged to be adjustable by varying the tap coefficient of the digital filter. The tap coefficient of the digital filter is arranged to be variable by means of a coefficient control circuit 511. The tap coefficient varying action is performed as follows: a subtracter 512 is arranged to receive the re-sampled image signal output from the automatic equalizer 510 and an ideal response waveform signal In generated by an oscillator which is not shown. The subtracter 512 produces a difference between the ideal response waveform signal In and the re-sample value of the phase reference signal part of the re-sampled image signal as a result of subtraction. In accordance with the result of the subtraction, the coefficient control circuit 511 forms and produces a tap coefficient selection signal for changing the tap coefficient of the digital filter included in the automatic equalizer 510. In accordance with this signal, the automatic equalizer 510 selects a tap coefficient most apposite to the digital filter. The re-sampled image signal is thus automatically Nyquist-equalized. The digital reproduced image signal which has been thus equalized is stored in an image memory 513.
The above-stated reproduced signal processing action is performed on the signal reproduced from each of the four tracks of the video floppy disc 501. A series of sample values which is shown in FIG. 2 and which corresponds to the offset sampled luminance signal shown in FIG. 1 is eventually stored in the image memory 513.
An interpolation circuit 514 then performs an interpolation process by using the sample data stored in the image memory 513. After the interpolation process, the sample data is read out from the image memory 513. The data read out is converted into an analog signal by a digital-to-analog (D/A) converter 515. The analog signal thus obtained is output through an LPF (low-pass filter) 516 as a reproduced image signal.
As to the phase reference signal to be added to the image signal, either an impulse type signal or a step type signal is employed. Further, to let the phase reference signal have an advantage over a noise or the like which might mix therewith, the phase reference signal is generally added at about a 100% level relative to the white level of the image signal. FIG. 5 shows the waveform of the phase reference signal reproduced by the reproducing operation in a case where the impulse type phase reference signal is added to the image signal in recording. If the step type phase reference signal is added to the image signal in recording, the waveform of the reproduced phase reference signal becomes as shown in FIG. 6. As shown in FIGS. 5 and 6, a signal of the type having a sudden level change brings about a ringing component which is a transient vibration. Such a ringing component occurs not only in the phase reference signal part but also in other parts where the level of the image signal suddenly changes.
Further, the image signal reproducing apparatus according to the CHSV system shown in FIG. 4 is arranged to equalize the waveform of the reproduced image signal by means of the automatic equalizer 510 after the image signal is sampled by the A/D converter 507. Therefore, the A/D converter 507 must be arranged to have a dynamic range which is sufficient for the reproduced image signal before the waveform equalization. In other words, the dynamic range of the A/D converter 507 must be set at a value C=(A+2.times.B) as shown in FIG. 6. Since the maximum possible value of a value B of FIG. 6 is about 0.2.times.A, the dynamic range of the A/D converter 507 must be set at a value which is about 1.4 times as large as the dynamic range A of the input signal.
Further, considering fluctuations of the filter characteristic of the aperture correction filter 505, changes in the frequency characteristic taking place according to the reproducing track position on the video floppy disc, etc., the dynamic range of the A/D converter 507 must be set at a value larger than the above-stated value. However, with the dynamic range of the A/D converter being thus broadened, it would increase a quantization noise if the number of quantization bits remains the same. For example, if the dynamic range of the A/D converter is set at a value which is about 1.4.times.A (A: the dynamic range of the input signal of the A/D converter), the quantization noise would increase by about 3 dB. The noise deteriorates the quality of the reproduced picture. This presents such a problem that the quantization bits are wasted.